The present invention proceeds from a device for evaluating periodic signals from inductive pick-ups which has a peak detector. The pick-up signal can be applied from the output of the inductive pick-up in order, as a function of the pick-up signal amplitude, to influence the transition from a first to a second logic state at the output of a downstream pulse former stage having two inputs. It is possible to feed the first input with a variable reference signal and the second input with the pick-up signal.
German Patent 2,235,056 discloses a circuit arrangement for an inductive pick-up. Variation in the external magnetic conditions due to parts that are moved past periodically induces an AC voltage in this pick-up. The circuit has a trigger circuit which then converts this AC voltage into a pulse train of the same frequency. For this purpose, the AC voltage is applied to the input of a peak detector which forms from the peak value a proportional direct voltage which is fed to the trigger circuit in order to influence its turn-on level such that the peak value of the AC voltage still just causes the trigger circuit to change state. A fundamental disadvantage of this circuit arrangement is that it does not eliminate the phase error which is caused by the frequency-dependent inductive reactive component of the internal impedance of the pick-up and is speed-dependent, which generally means frequency-dependent.
The printed publication "Motor-Elektronik", 1983 published by Robert Bosch GmbH, discloses a device for evaluating periodic signals from inductive pick-ups. An example of such a pick-up described there is a crankshaft angle pick-up which transmits a speed-analog AC voltage. In order to be able to process this AC voltage in an electronic control unit, the AC voltage is converted by means of a Schmitt trigger into square-wave current pulses; the fixed hysteresis of the Schmitt trigger simultaneously increases in this arrangement the insensitivity with respect to specific disturbances. Disturbances which reach beyond the hysteresis threshold of the Schmitt trigger lead, however, to missing pulses at the output of the Schmitt trigger.
German Offenlegungsschrift 3,433,777 describes an improvement in this regard in which a trigger threshold of the Schmitt trigger is shifted as a function of the signal of a peak detector. The result is an adaptive pulse shaping which depends on different pick-up amplitudes and is intended to offer an improved interference immunity. Furthermore, it is proposed in the case of a desirable phase-accurate detection of the pick-up signal permanently to set a trigger threshold of the Schmitt trigger, in particular at the zero crossing of the pickup voltage.
A disadvantage of this known device is that it does not eliminate the frequency-dependent phase error mentioned above. In this device, a "phase-accurate detection" is possible respectively at a specific speed of a rotation element to be monitored or at a fixed frequency of the pick-up signal only to the extent that this is taken to mean an unambiguous coordination between the instantaneous voltage and position of the rotation element whose rotary position is to be scanned.
It is an object of the invention to provide a device for evaluating periodic signals from inductive pick-ups which have a substantially reduced phase error in the overall nominal frequency range of periodic input signals.
This and other objects are achieved by the present invention which provides a device for evaluating periodic signals from inductive pick-ups. This device has a peak detector to which a pick-up signal can be applied from an output of the inductive pick-up in order to influence, as a function of the pick-up signal amplitude, a transition from a first to a second logic state at an output of a downstream pulse former stage having first and second inputs. The first input is provideable with a variable reference signal and the second input with the pick-up signal. A pulse generator stage receives the pick-up signal as an input and generates at an output pulses at a repetition frequency which corresponds to the frequency of the pick-up signal. A weighting stage combines the signals from the output of the peak detector and from the output of the pulse generator to form a variable reference signal in a predetermined fashion.
An important advantage of the device according to the present invention is that the lagging of the periodic signal of an inductive pick-up in the case of increasing frequency, for example in the case of increasing speed of a part that is to be continuously scanned, is compensated for. As a result, for example, the absolute reference error between the actual position of the part to be scanned and the instantaneous value, characterizing this position, of the pick-up signal is reduced. An integral part of this process, which further increases the reference accuracy, is an exertion of influence, on a signal edge provided for further processing, as a function of the amplitude of the pick-up signal.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.